Electrophotographic lithographic original plates (hereinafter referred to as "masters") have a photosensitive layer comprising fine photoconductive particles, such as zinc oxide particles, dispersed in a resin binder, and are obtained by forming an ink-receptive image on this layer by an ordinary electrophotographic technique.
In lithographic printing, a plate having nonimage areas (hydrophilic areas) readily wettable by water and image areas (ink-receptive areas) sparingly wetted by water is generally used. However, in the case of electrophotographic original lithographic printing plates, normal printing is impossible when the printing plates are used as they are, because the printing surface of these untreated plates is made of a hydrophobic photoelectroconductive layer and a printing ink hence adheres also to nonimage areas.
It is therefore necessary to desensitize the nonimage areas of such an original printing plate prior to printing to impart hydrophilicity. Proposed so far as this kind of desensitizing solutions include a cyanide-containing desensitizing solution containing a ferrocyanate and a ferricyanate as major components and a cyanide-free desensitizing solution containing an ammine/cobalt complex, phytic acid (inositol hexaphosphate), a derivative of the acid, and a guanidine derivative as major components.
However, these prior art desensitizing solutions are not wholly satisfactory. Specifically, the former desensitizing solution, which contains a ferrocyanate and a ferricyanate, has a drawback that it suffers discoloration and precipitation upon exposure to light because of the instability of ferrocyanate ions and ferricyanate ions to heat and light to come to have weakened desensitizing power, although it has high initial desensitizing power and is capable of rapidly forming a tenacious hydrophilic film. The former desensitizing solution has another drawback that since it contains cyanide (CN.sup.-) ions, a free cyanide is detected in wastewater, etc. to pose various problems concerning environmental pollution.
On the other hand, the latter desensitizing solution, which is a cyanide-free desensitizing solution containing desensitizing agents such as an ammine/cobalt complex, phytic acid, and guanidine as major components, was proposed in view of the drawbacks described above. However, this prior art desensitizing solution also cannot give a fully satisfactory lithographic original plate. Specifically, the latter desensitizing solution has a drawback that since it has a lower film-forming rate than the former cyanide-containing desensitizing solution, a hydrophilic film having high physical strength and capable of being immediately subjected to printing cannot be formed when an original plate is etched only once with the latter desensitizing solution in a processor, leading to scumming and plugging of halftone dot.
It has conventionally been well known that phytic acid and metallized derivatives thereof form metal chelate compounds, and various proposals have been made on use of these compounds as desensitizing agents for original offset printing plates. However, all these desensitizing agents have a drawback that since they have a low film-forming rate, a hydrophilic film usable in printing cannot be formed through one treating operation in a processor and the resulting film has poor ink repellency, leading to scumming and plugging of halftone dot.
For eliminating the problems described above, investigations are being made on addition of various additives to desensitizing solutions based on phytic acid.
Examples thereof include a desensitizing solution containing a combination of phytic acid and a metal complex of an aminocarboxylic acid or the like (see JP-B-2-39397; the term "JP-B" as used herein means an "examined Japanese patent publication"), a desensitizing solution containing a combination of phytic acid and a hexametaphosphate (see JP-B-62-7597), and desensitizing solutions containing a lower amine, an alkanolamine, or a polyamine (see, for example, JP-A-54-117201, JP-A-53-109701, and JP-A-1-25994; the term "JP-A" as used herein means an "unexamined published Japanese patent application"). Although these desensitizing solutions are satisfactory in water receptivity in the initial stage of use, a sufficient effect cannot be obtained therewith because they have problems, for example, that continuous use results in reduced etching and reduced water receptivity and use after long-term storage results in reduced water receptivity to cause scumming.
Desensitizing solutions containing a cationic polymer (see, for example, JP-A-60-23099) have drawbacks that continuous use and long-term storage result in a decrease in performance as in the above-described desensitizing solutions, and that they cause rusting.
Further, desensitizing solutions containing a combination of phytic acid and a polyethyleneimine copolymer have been proposed (see, for example, JP-A-7-68967 and JP-A-7-137475). However, this kind of desensitizing solutions still have problems, for example, that the latitude in which the impartation of hydrophilicity to nonimage areas by etching is consistent with the impartation of ink receptivity to image areas is narrow, or long-term continuous use results in a decrease in performance.
On the other hand, automatic printing machines especially of small size which have a desensitizing system united therewith have spread increasingly in recent years from the standpoint of labor reduction. In addition, the time required for electrophotographically produced offset masters to be processed to give finished printing plates is being reduced. Under these circumstances, a desensitizing treatment is required to be carried out rapidly and to meet the attainment of a longer life.
With respect to systems for electrophotographically producing masters, a digital exposure technique has been proposed. As a result, not only conventional masters bearing images consisting mainly of line originals and characters, but also masters bearing fine images such as middle tone images, screen tints, etc. have come to be easily produced. Although printing plates are hence required to reproduce such fine images on prints, this is difficult to attain with any of the prior art known desensitizing solutions.